Ada Programming, Algorithms, Knuth-Morris-Pratt pattern matcher

File: Algorithms/pattern_match_knuth_morris_pratt_test.adb (view, plain text, download page, browse all)

-- pattern_match_knuth_morris_pratt_test.adb an implementation for fixed strings

-- Written by Wikibob, 2004, from notes on the Knuth_Morris_Pratt pattern match algorithm
-- adapted to fixed strings of characters.
-- It is in the public domain.

-- If you are using GNAT, use gnatmake to compile and link this program.
-- To use the pattern match functions in your own software extract
-- the inner package's specification and body into separate files.

-- This program is self-contained and demonstrates a particular
-- implementation of the Knuth_Morris_Pratt algorithm applied to
-- fixed strings, with the following restrictions:
-- * the search pattern is limited to a maximum of 256 characters
-- * the caller must first call the function Pre_Compute on a pattern
-- to obtain a context variable containing the pre-computed pattern.
-- There is no limit to the number of contexts.
-- * the caller must handle the exception Pattern_Error that will
-- be raised if function Find_Location was unable to find the
-- pattern in the given string.

-- Suggested improvements to the inner package are:
-- * add type Result_T is record Location : Index; Found : Boolean; end record;
-- and use it instead of raising Pattern_Error.
-- * produce a version that dispenses with the Context and has Find_Location
-- perform the Pre_process internally.

-- References: http://ww0.java3.datastructures.net/handouts/PatternMatching.pdf

procedure Pattern_Match_Knuth_Morris_Pratt_Fixed_Test is
-- You may extract this spec into file pattern_match.ads
package Pattern_Match is
Max_Pattern_Length : constant Positive := 256;
type Context is private;
function Pre_Compute (Pattern : in String) return Context;
-- precomputes the table of skips for the Pattern.
function Find_Location (Of_Context : in Context;
In_Text : in String) return Positive;
Pattern_Error : exception;
-- alternative is return Natural and use 0 to mean not found.
private
subtype Pattern_Length_T is Positive range 1..Max_Pattern_Length;
type Failure_Function_T is array (Pattern_Length_T) of Positive;
subtype Slided_Pattern_T is String (1 .. Max_Pattern_Length);
type Context is record
Failure_Function : Failure_Function_T;
M_Pattern : Slided_Pattern_T;
Pattern_Length : Positive;
end record;
end Pattern_Match;

-- Variables and data for testing.
IFPLID_Context : Pattern_Match.Context;
SRC_Context : Pattern_Match.Context;
Text_Test1 : constant String := "IMCHG DLH5877 -BEGIN ADDR -IFPLID AT05428113 -SRC FPL -RFL F330";
Text_Test2 : constant String := "IMCHG DLH5877 EDDKCLHD -BEGIN ADDR -FAC CFMUTACT AA05428113 FPL -STAR WLD5M -SRC ";
IFPLID_Pos : Positive;
IFPLID_Pos_2 : Positive := 1;
SRC_Pos : Positive;
SRC_Pos_2 : Positive;

-- You may extract this spec into file pattern_match.adb
package body Pattern_Match is
function Pre_Compute (Pattern : in String) return Context is
I, J : Positive;
Pattern_Context : Context;
begin
if Pattern = "" then
raise Pattern_Error;
end if;
Pattern_Context.M_Pattern (1..Pattern'Length) := Pattern;
Pattern_Context.Pattern_Length := Pattern'Length;
Pattern_Context.Failure_Function (1) := 1;
I := 2;
J := 1;
while I <= Pattern_Context.Pattern_Length loop
if Pattern (I) = Pattern (J) then
-- we have matched J + 1 chars.
Pattern_Context.Failure_Function (I) := J + 1;
I := I + 1;
J := J + 1;
elsif J > 1 then
-- use failure function to shift Pattern
J := Pattern_Context.Failure_Function (J - 1);
else
Pattern_Context.Failure_Function (I) := 1;
I := I + 1;
end if;
end loop;
return Pattern_Context;
end Pre_Compute;

function Find_Location (Of_Context : in Context;
In_Text : in String) return Positive is
subtype Slided_Text_T is String (1 .. In_Text'Length);
Slided_Text : constant Slided_Text_T := Slided_Text_T (In_Text);
I, J : Positive;
begin
I := 1;
J := 1;
while I <= Slided_Text'Last loop
if Slided_Text (I) = Of_Context.M_Pattern (J) then
if J = Of_Context.Pattern_Length then
return I - J + 1;
else
I := I + 1;
J := J + 1;
end if;
elsif J > 1 then
J := Of_Context.Failure_Function (J - 1);
else
I := I + 1;
end if;
end loop;
raise Pattern_Error;
-- Or change function to return Natural and return 0.
end Find_Location;
end Pattern_Match;

-- You may extract the rest of this file into file pattern_match_test.adb
-- and modify accordingly.

procedure Check_Pattern_Found (Pattern : in String;
At_Location : in Positive;
In_Text : in String) is
subtype Slided_Text_T is String (1 .. Pattern'Length);
Slided_Pattern : constant Slided_Text_T := Slided_Text_T (Pattern);
begin
if At_Location > In_Text'Last or else
At_Location + Pattern'Length - 1 > In_Text'Last or else
Slided_Text_T (In_Text (At_Location .. At_Location + Pattern'Length - 1)) /= Slided_Pattern
then
-- We expected Find_Location to return the location of the pattern, as it did not there is a program error.
raise Program_Error;
end if;
end Check_Pattern_Found;

begin
IFPLID_Context := Pattern_Match.Pre_Compute ("-IFPLID ");
SRC_Context := Pattern_Match.Pre_Compute ("-SRC ");

Expect_Pattern_Found:
begin
IFPLID_Pos := Pattern_Match.Find_Location (Of_Context => IFPLID_Context,
In_Text => Text_Test1);
exception
when Pattern_Match.Pattern_Error =>
-- We expected Find_Location to find the pattern, but it did not so there is a program error.
raise Program_Error;
end Expect_Pattern_Found;
Check_Pattern_Found (Pattern => "-IFPLID ",
At_Location => IFPLID_Pos,
In_Text => Text_Test1);

Expect_Pattern_Not_Found:
begin
IFPLID_Pos_2 := Pattern_Match.Find_Location (Of_Context => IFPLID_Context,
In_Text => Text_Test2);
-- We expected Find_Location to NOT find the pattern, but it did so there is a program error.
raise Program_Error;
exception
when Pattern_Match.Pattern_Error =>
-- We expected Find_Location to NOT find the pattern, and it did not so there is no error.
null;
end Expect_Pattern_Not_Found;
if IFPLID_Pos_2 /= 1 then
-- We expected Find_Location to NOT return a result, so there is a program error.
raise Program_Error;
end if;

Expect_Second_Pattern_Found:
begin
SRC_Pos := Pattern_Match.Find_Location (Of_Context => SRC_Context,
In_Text => Text_Test1);
exception
when Pattern_Match.Pattern_Error =>
-- We expected Find_Location to find the pattern, but it did not so there is a program error.
raise Program_Error;
end Expect_Second_Pattern_Found;
Check_Pattern_Found (Pattern => "-SRC ",
At_Location => SRC_Pos,
In_Text => Text_Test1);

Expect_Second_Pattern_Found_At_End:
begin
SRC_Pos_2 := Pattern_Match.Find_Location (Of_Context => SRC_Context,
In_Text => Text_Test2);
exception
when Pattern_Match.Pattern_Error =>
-- We expected Find_Location to find the pattern, but it did not so there is a program error.
raise Program_Error;
end Expect_Second_Pattern_Found_At_End;
Check_Pattern_Found (Pattern => "-SRC ",
At_Location => SRC_Pos_2,
In_Text => Text_Test2);

end Pattern_Match_Knuth_Morris_Pratt_Fixed_Test;

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